Polybutylene terephthalate (PBT) reinforced with thermally stable reinforcing fibers such as glass fibers is well known as a molding resin and is described in numerous patents and publications including for instance U.S. Pat. Nos. 2,814,725, 4,124,561, 3,814,786 and 3,625,024. Fiber reinforcement generally improves the tensile strength, flexural strength, flexural modulus and heat distortion temperature of the molding composition. However, moldings, especially injection moldings of large fiber glass reinforced articles of PBT, nylon and other semicrystalline thermoplastics tend to display distortion or warping while glass fiber reinforced armorphous thermoplastic compounds do not present such problems. It is believed that strains resulting from the different degrees of volumetric contraction parallel to and transverse to the direction of plastic melt flow into the mold during the cooling of molded articles are responsible for such warping. Orientation of the glass fibers parallel to the direction of melt flow during molding produces this directional difference in volumetric contraction. The warping is thus believed due to the presence of the very reinforcing fibers which contribute to the enhanced physical characteristics of the finished product. It is known that addition of mica to fiberglass reinforced PBT reduces warping. Unfortunately, the mica also greatly reduces impact strength.
Various impact modifiers are also known which improve the impact strength of molded PBT compositions. Some of these are described for instance in U.S. Pat. Nos. 4,096,202 and 4,034,013. It is generally believed and unfortunately true, that some modifiers which improve impact characterists of PBT or other poly (C.sub.2 -C.sub.4 alkylene terephthalate) molding compositions, including fiber reinforced compositions, also tend to increase the warping characteristics of the compositions.